1. Field of the Invention
The present invention relates to an oil type release agent for die casting, a method for setting a solvent mixing ratio, a casting method using the oil type release agent, and a spray unit. The invention is also applicable for a plunger chip as a lubricant.
2. Description of the Related Art
As is well known, in die casting, to lubricate the cavity part of a die, an oil film is formed on the cavity surface of the die by spraying a lubricant called as a release agent after die opening. The oil film prevents a cavity from soldering of a non-ferrous molten metal of such as aluminum, magnesium and zinc on the cavity and makes possible casting continuously. The release agents for die casting are broadly classified into the oil type release agent and a water soluble type release agent. In terms of the productivity, the safety, and the work environments, the water soluble type release agent has often been used in recent years.
However, before 40 years ago, a release agent was only oil type (hereinafter, referred to as old oil type release agent) containing solid matters of lard, powder and graphite with which machinery is made sticky after use. Users diluted the agent with an economical kerosene or solvent and sprayed the diluted agent. However since the old oil type release agent contained a powder, the powder scattered in the peripheral areas of the die during casting, worsened the work environments and deposited on the die. Thus frequent cleaning was indispensable. The old oil type release agent was mixed with kerosene which had a low flash point. It was so risky as to cause a fire and thus made automation of die casting difficult. Because of this reason, the old oil type release agent was applied manually, resulting in low production efficiency. Further, since the refining degree of the kerosene was low and trace components such as sulfur or the like were contained, the agent might inevitably cause adverse effects on the human body and emitted intense oily smell. That is, the old oil type release agent had risks of fire and explosion, was unsuitable for automation, polluted the working environments with oil and powders, and inevitably required periodic cleaning works.
Because of the above described situations, the old oil type release agent was changed to the water soluble type release agent with less risk of firing for automation. It is no exaggeration to say that 99% of release agents made commercially available are water-soluble-type release agents now. On the other hand, very a few oil type release agents containing no solid matter (hereinafter, referred to simply as highly viscid oil type release agent) have continuously been used. Such oil type release agents have an excellent lubricating property. However, the viscosity is very high (dynamic viscosity at 40° C. is 100 mm2/s or higher) and even if they are sprayed, the produced mist has a large diameter. Therefore, the agents are unsuitable for automatic spraying and consequently consumed much, and the oil components of them are entrained and gasified in the flow of molten metal to remain in the form of gas in cast products and accordingly result in increase of the porosity. Consequently, utilizing the excellent lubricating property of them, the old oil type release agents have presently been used only for warm-up operation before applying the water soluble type release agent.
On the other hand, the water soluble type release agent free from the risk of the fire has a crucial defective point in the capability. Since the agent is diluted with water about 80 times as much at the time of use, 99% of the main component are water and therefore the agent causes Leidenfrost phenomenon on the die at around 150° C. That is, the release agent mist is explosively evaporated around 150° C. and the die surface is covered with a steam film. Therefore the release agent mist, which comes next, cannot arrive at the die surface. This causes the decrease of the adhesion amount of active components in the release agent on the die surface. To increase the adhesion amount, the die temperature is kept below the Leidenfrost temperature by spraying a large quantity of the water soluble type release agent while scarifying the adhesion efficiency. As a matter of fact, presently the spray amount is approximately the same as the number of the tons of the locking force of a casting machine (e.g. about 350 cc for a 350 t-machine, about 2500 cc for a 2500 t-machine). Naturally, the peripheral areas of the machine become dirty, the waste fluid is much, thereby it is required much labor and cost for cleaning and waste fluid treatment. Also, since almost all the water soluble type release agents contain waxes, solidified waxes adhere to the die surface and deposit to peripheral areas of the machine. It requires frequent cleaning. Not only the pollution in the peripheral areas of the machine due to the precipitation and adhesion of the release agent's components, but also oxidation deterioration of the components has to be taken into consideration. Patent Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 8-103913) describes the use of an oxidation prevention agent for suppressing oil component deterioration in the water soluble type release agent. The invention aims the pollution prevention of a die in a rubber vulcanization process. And it also discloses a countermeasure for apparently decreasing stains on the die.
Further, the die is heated to about 200 to 350° C. with aluminum molten metal every shot and thereafter cooled to about 100 to 150° C. with the water soluble type release agent. The temperature of the die surface fluctuates from 100 to 200° C. in every shot. Consequently, after continuous casting for a long duration (several thousand times for a large scale die and several ten thousand times for a small scale die), thermal fatigue is accumulated in the die surface, so-called cracks are formed and finally the costly die is broken. This is the present situation.
Further, since the water soluble type release agent has a strong cooling capability, the molten aluminum injected into the cavity is cooled within a short time. The viscosity of the molten metal is increased to disturb the molten metal flow. Finally the molten metal can not reach to every fine corner of the cavity. As a result, so-called “misrun” and “shrinkage” phenomena occur and make it impossible to produce a complete cast product. Also, since the adhesion efficiency of the water soluble type release agent is low, the oil film on the metal surface is thin. Soldering may often occur at high temperature portions of the die, especially thin parts like core pins.
Porosity, which decreases the strength of the cast product, is also a problem. The cause of the porosity is to entrain organic matters and water into turbulent flow of the molten metal and to gasify in the casting product. If an excessive amount of the release agent is sprayed, the porosity increases. In the past, to lower the porosity, Patent Document 2 (Jpn. Pat. Appln. KOKAI Publication No. 2000-33457) disclosed a powder type release agent having excellent releasing capabilities.
In the above-mentioned current situation, it has been desired to improve the disadvantageously low adhesion efficiency of the water soluble type release agent, to improve the spraying property of highly viscus oil type release agent while keeping the excellent lubrication property, and to make it possible to achieve “very small amount of spray”, “long die lie” and “less waste fluid”.